The field of this invention relates to use of heterogeneous adsorbents in purification of relatively impure olefins such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks. More particularly, this invention concerns purification by passing an olefinic process stream, containing small amounts of acetylenic impurities, and optionally carbon oxides and/or other organic components which are, typically, impurities in cracked gas, through a particulate bed of heterogeneous adsorbent comprising a metal supported on a high surface area carrier, under conditions suitable for adsorption of alkynes. Beneficially, a limited amount of dihydrogen is fed into the bed during treatment of the olefinic process stream thereby increasing capacity of the adsorbent for acetylenics.
Beneficially, a sub-stoichiometric amount of dihydrogen is admixed with the olefinic process stream fed into the bed thereby increasing capacity of the adsorbent for acetylenics.
Processes according to this invention are particularly useful where the olefin being purified is ethylene and/or propylene formed by thermal cracking of hydrocarbon feedstocks.
As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Olefins including, importantly, ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.
Commercial production of olefins is, almost exclusively, accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feed stocks include streams of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Because of the very high temperatures employed, commercial olefin processes invariably coproduce significant amounts of acetylene and methyl acetylene. Required separation of the acetylene from the primary olefin can, considerably, increase the plant cost.
In a typical ethylene plant the cracking section represents about 25 percent of the cost of the unit while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining about 75 percent of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large and ways to reduce it are of substantial commercial interest. In addition, it is of interest to reduce the amount of methane and heavy fuel oils produced in the cracking processor to utilize it other than for its fuel value.
Hydrocarbon cracking is carried out using a feed which is ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Ethane, propane, liquid naphthas, or mixtures thereof are preferred feed to a hydrocarbon cracking unit. Hydrocarbon cracking is, generally, carried out thermally in the presence of dilution steam in large cracking furnaces which are heated by burning, at least in part, methane and other waste gases from the olefins process resulting in large amounts of NOx pollutants. The hydrocarbon cracking process is very endothermic and requires large quantities of heat per pound of product. However, newer methods of processing hydrocarbons utilizes at least to some extent catalytic processes which are better able to be tuned to produce a particular product slate. The amount of steam used per pound of feed in the thermal process depends to some extent on the feed used and the product slate desired. Typically, steam pressures are in the range of about 30 lbs per sq in to about 80 lbs per sq in, and amounts of steam used are in the range of about 0.2 pounds of steam per pound of feed to 0.7 pounds of per pound of feed. The temperature, pressure and space velocity ranges used in thermal hydrocarbon cracking processes to some extent depend upon the feed used and the product slate desired which are well known as may be appreciated by one skilled in the art.
The type of furnace used in the thermal cracking process is also well known. However the ceramic honeycomb furnace which is described in U.S. Pat. No. 4,926,001, the contents of which patent are specifically incorporated herein by reference, is an example of a new type of cracking which could have a special utility for this process.
Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid adsorption, membrane separation and the so called xe2x80x9cpressure swing adsorptionxe2x80x9d in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Cryogenic distillation and liquid adsorption are common techniques for separation of carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g., nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid adsorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Molecular sieves which selectively adsorb carbon monoxide from gaseous mixtures by chemisorption are also known. U.S. Pat. Nos. 4,019,879 and 4,034,065 refer to use of high silica zeolites, which have relatively high selectivities for carbon monoxide, in the pressure swing adsorption method. However, these zeolites only have moderate capacity for carbon monoxide and more particularly require very low vacuum pressures to recover the adsorbed gases and/or to regenerate the zeolite.
U.S. Pat. No. 4,717,398 describes a pressure swing adsorption process for selective adsorption and subsequent recovery of an organic gas containing unsaturated linkages from gaseous mixtures by passing the mixture over a zeolite ion-exchanged with cuprous ions (Cu I) characterized in that the zeolite has a faujasite type crystalline structure (Y).
U.S. Pat. No. 6,215,037 in the name of Joel Padin, Curtis L. Munson and Ralph T. Yang provides a selection of specific zeolites said to be useful for selective adsorption of dienes from mono-olefins. In particular, the adsorbents are ion-exchanged zeolites of the group consisting of zeolite X, Zeolite Y and zeolite LSX in a form having exchangeable cationic sites. According to the patent, essentially all cationic sites of the ion-exchanged zeolite must contain silver cation or copper cation for the selective separation of diene from mono-olefin which the same number of carbon atoms.
Kokai JP Number 50929-1968 describes a method of purifying vinyl compounds containing up to about 10 percent by weight of acetylene compounds including ethyl acetylene, vinyl acetylene and phenyl acetylene whereby the acetylene compounds are adsorbed in an adsorption agent of 1-valent and/or 0-valent copper and/or silver supported on inert carrier such as xcex4-alumina, silica or active carbon. However, it is well known that acetylene and these acetylene compounds react with copper and/or silver to from copper acetylide or silver acetylide. Both the acetylide of copper and silver are unstable compounds. Because they are explosive under some conditions their possible formation presents safety problems in operation and in handling adsorbent containing such precipitates.
German Disclosure Document 2059794 describes a liquid adsorption process for purification of paraffinic, olefinic and/or aromatic hydrocarbons with an adsorption agent consisting in essence of a complex of a copper (Cu I)-salt with an alkanolamine such as mono-ethanolamine, mono-isopropanolamine, di-ethanolamine, tri-ethanolamine and arylalkanolmines, and optionally in the presence of a glycol or polyglycol. However, the product stream is contaminated with unacceptable levels of components of the such agents absorbed in the hydrocarbon flow. While such contamination might be removable using an additional bed of silica gel, aluminum oxide or a wide-pored molecular sieve, this would involve additional capital costs, operation expenses and perhaps safety problems.
Processes using heterogeneous adsorbents are known for purification of olefins, such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks, by passing a stream of olefin through a particulate bed of support material on which is dispersed a metallic element. U.S. Pat. Nos. 6,080,905 and 6,124,517 in the name of Mark P. Kaminsky, Shiyou Pei, Richard A. Wilsak, and Robert E. Whittaker describe adsorption which is carried out in an essentially dihydrogen-free atmosphere within the bed. Adsorption of the contained acetylenic impurities is continued until levels of acetylenic impurities in the effluent stream increase to a predetermined level. Thereafter the resulting bed of adsorbent is regenerated using hydrogen to effect release of the contained acetylenic impurities from the adsorbent. However, there remains a need to increase the capacity of adsorbents for acetylenics whereby the useful life of the adsorbent bed between regenerations is increased.
Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation is carried out at about xe2x88x9225xc2x0 C. and 320 pounds per square inch gauge pressure (psig) in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about xe2x88x9230xc2x0 C. and 30 psig.
Impurity refers to compounds that are present in the olefin plant feedstocks and products. Well-defined target levels exist for impurities. Common impurities in ethylene and propylene include: acetylene, methyl acetylene, methane, ethane, propane, propadiene, and carbon dioxide. Listed below are the mole weight and atmospheric boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.
Recently the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. Need for purity improvements is directly related to increasing use of higher activity catalysts for production of polyethylene and polypropylene, and to a limited extent other olefin derivatives.
It is known that acetylenic impurities can be selectively hydrogenated and thereby removed from such product streams by passing the product stream over an acetylene hydrogenation catalyst in the presence of dihydrogen (molecular hydrogen, H2). However, these hydrogenation processes typically result in the deposition of carbonaceous residues or xe2x80x9cgreen oilxe2x80x9d on the catalyst which deactivates the catalyst. Therefore, acetylene hydrogenation processes for treating liquid or liquefiable olefins and diolefins typically include an oxygenation step or a xe2x80x9cburnxe2x80x9d step to remove the deactivating carbonaceous residues from the catalyst followed by a hydrogen reduction step to reactivate the hydrogenation catalyst. For example, see U.S. Pat. No. 3,755,488 to Johnson et al., U.S. Pat. No. 3,792,981 to Hettick et al., U.S. Pat. No. 3,812,057 to Morgan and U.S. Pat. No. 4,425,255 to Toyoda. However, U.S. Pat. Nos. 3,912,789 and 5,332,705 state that by using selected hydrogenation catalysts containing palladium, at least partial regeneration can be accomplished using a hydrogenation step alone at high temperatures (600xc2x0 F.-700xc2x0 F.) and in the absence of an oxygenation step.
Selective hydrogenation of the about 2000 to 4000 parts per million of acetylenic impurities to ethylene is, generally, a crucial operation for purification of olefins produced by thermal steam cracking. Typical of a small class of commercially useful catalysts are materials containing very low levels of an active metal supported on an inert carrier, for example a particulate bed having less than about 0.03 percent (300 ppm) palladium supported on the surface skin of carrier pellets having surface area of less than about 10 m2/gm.
Many commercial olefin plants using steam crackers use, generally, front-end acetylene converters, i.e., the hydrogenation unit is fed C3 and lighter cracked gas which feed has a high enough concentration of hydrogen to easily hydrogenate the acetylenic impurities, however, when run improperly, will also hydrogenate a large fraction of the ethylene and propylene product. Both hydrogenation of acetylene and ethylene are highly exothermic as shown below:
Accelerated catalyst deactivation and thermal runaways caused by loss in catalyst selectivity are common problems which plague acetylene converters. Such problems result in unscheduled shutdowns and increased costs to replace deactivated catalyst.
The problem of over-hydrogenation is aggravated because the rate constant for ethylene hydrogenation to ethane is 100 times faster than for the hydrogenation of acetylene to ethylene. As a means to avoid a C2 H4 hydrogenation thermal runaway, acetylene, carbon monoxide and diolefins concentrations must, therefore, be high enough to cover most active sites so none are left to adsorb ethylene. For example, acetylene, carbon monoxide, methyl acetylene, and propadiene have bond strengths to palladium which are stronger than the ethylene to palladium bonds. Selection of active metal, size of the metal particles and other physical and chemical factors ultimately affect the xe2x80x9coperating temperature windowxe2x80x9d which is the delta of temperature between acetylene conversion to ethylene (typically in a range from about 100xc2x0 F. to about 150xc2x0 F.) and thermal runaway where all molecular hydrogen is converted and a large amount of the ethylene is converted to ethane (about 170xc2x0 F. to about 225xc2x0 F.). The wider the window, the safer is operation of the unit.
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods, for production of olefins from thermal cracking of hydrocarbon feed stocks which olefin can be used for manufacture of polymeric materials using higher activity catalysts.
More particularly, it is an object of the present invention to provide an improved method for purification of ethylene and/or propylene containing small amounts of acetylenic impurities, carbon oxides and/or other organic components that are impurities in olefinic process streams, by passing the impure olefin stream through a particulate bed of heterogeneous adsorbent comprising a metal supported on a high surface area carrier, under conditions suitable for adsorption of alkynes impurities.
It is another object of the present invention to provide an improved aforesaid purification method that employs an adsorbent that, even after a substantial period of aging, exhibits ability to withstand repeated regenerations and yet retain useful adsorption capacity. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.
Economical processes are disclosed which obtain purified olefinic product by purification of a relatively impure olefins such as are produced, for example, by thermal cracking of hydrocarbons. Processes of this invention comprise providing a impure mixture comprising at least one olefin of from 2 to about 8 carbon atoms, acetylenic impurities having the same or similar carbon content in an amount of up to about 1 percent by volume base upon the total amount of olefin present and optionally saturated hydrocarbon gases; admixing a source of dihydrogen with the impure mixture to form a feedstream comprising a sub-stoichiometeric amount of dihydrogen based upon conversion of the total amount of acetylenic impurities present to their olefinic analogs; passing the feedstream through a particulate bed of adsorbent comprising predominantly a support material having high surface area on which is dispersed at least one metallic element in the zero valent state selected from the group consisting of chromium, iron, cobalt, nickel, copper, ruthenium, palladium, silver and platinum, to effect, under conditions suitable for adsorption within the bed, selective adsorption and/or complexing of the contained acetylenic contaminants with the adsorbent, and thereby obtain purified olefinic product which contains less than about 1 part per million by volume of the acetylenic impurities; and thereafter regenerating the resulting bed of adsorbent in the presence of a reducing gas comprising dihydrogen (molecular hydrogen) to effect release of the contained acetylenic impurities from the adsorbent.
It has been found that the presence of a small amount of dihydrogen in an olefinic feedstream containing acetylenic impurities unexpectedly increases the capacity of the adsorbent for the acetylenics therein. The amount of dihydrogen present in the feedstream beneficially is less than that amount necessary to stoichiometrically hydrogenate all acetylenic impurities to their corresponding olefin.
Another aspect of special significance is the separation of acetylenic impurities from ethylene or propylene containing small amounts of acetylene, i.e., less than about 5000 parts per million by weight of one or more acetylenic impurities, and provide, advantageously, purified product containing less than about 1 parts per million by weight, and frequently even less than about 0.5 parts per million by weight.
In yet another aspect the invention is a process for purification of olefins produced by thermal cracking of hydrocarbons which comprises: providing a impure mixture comprising at least about 99 percent by volume of an olefin having from 2 to about 4 carbon atoms, and acetylenic impurities having the same or similar carbon content in an amount in a range upward from about 1 to about 1000 parts per million by volume; admixing a source of dihydrogen with the impure mixture to form a feedstream comprising a sub-stoichiometric amount of dihydrogen based upon conversion of the total amount of acetylenic impurities present to their olefinic analogs; passing the feedstream through a particulate bed of adsorbent comprising predominantly a support material selected from the group alumina, silica, active carbon, clay and zeolites having surface area in a range of from about 10 to about 2,000 square meters per gram as measured by the BET gas adsorption method, on which is dispersed at least one metallic element selected from the group consisting of chromium, iron, cobalt, nickel, copper, ruthenium, palladium, silver and platinum, to provide an effluent stream from the bed; effecting, under conditions suitable for adsorption within the bed, selective adsorption and/or complexing of the contained acetylenic contaminants with the adsorbent, until levels of the acetylenic impurities in the effluent stream increase to a predetermined level in a range downward from about 1 parts per million by volume; and thereafter regenerating the resulting bed of adsorbent in the presence of a reducing gas comprising dihydrogen to effect release of the contained acetylenic impurities from the adsorbent.
A preferred class of adsorbents useful in processes according the invention, comprises at least about 90 weight percent of a gamma alumina having surface area in a range of from about 80 to about 500 square meters per gram as measured by the BET gas adsorption method, and contains less than 500 parts per million by weight of a sulfur-containing component, calculated as elemental sulfur. More preferred are the adsorbent which comprises at least about 90 weight percent of a gamma alumina having surface area in a range of from about 150 to about 350 square meters per gram as measured by the BET gas adsorption method, and wherein the metal dispersed on the support material is palladium, and the absorbent has a palladium content in a range of from about 0.01 to about 10 percent based on the total weight of the adsorbent.
For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention.